Method for electronically tuning the readout vibration of a coriolis gyroscope

ABSTRACT

In a method for electronic tuning of the frequency of the read oscillation to the frequency of the stimulation oscillation in a Coriolis gyro, the resonator of the Coriolis gyro has a disturbance force applied to it such that the stimulation oscillation remains essentially uninfluenced. The read oscillation is changed so that a read signal that represents the read oscillation contains a corresponding disturbance component. The frequency of the read oscillation is controlled so that the magnitude of the disturbance component contained in the read signal is a minimum.

BACKGROUND

1. Field of the Invention

The present invention relates to Coriolis gyros. More particularly, thisinvention pertains to a method for electronic tuning of read oscillationfrequency to stimulation oscillation frequency in such a device.

2. Description of the Prior Art

Coriolis gyros (also known as “vibration gyros”) are increasinglyemployed for navigation. Such devices include a mass system that iscaused to oscillate. Such oscillation is generally a superimposition ofa large number of individual oscillations. The individual oscillationsof the mass system are initially independent of one another and each maybe regarded in the abstract as a “resonator”. At least two resonatorsare required for operation of a vibration gyro. A first resonator isartificially stimulated to oscillate, with such oscillations referred tobelow as a “stimulation oscillation”. A second resonator is stimulatedto oscillate only when the vibration gyro is moved or rotated. That is,Coriolis forces occur which couple the first resonator to the secondresonator, draw energy from the stimulation oscillation of the firstresonator, and transfer the energy to the read oscillation of the secondresonator. The oscillation of the second resonator is referred to belowas the “read oscillation”. In order to determine movement (in particularrotation) of the Coriolis gyro, the read oscillation is tapped off and acorresponding read signal (e.g. the tapped-off read oscillation signal)is analyzed to determine whether any changes occurred in the amplitudeof the read oscillation that measures rotation of the Coriolis gyro.Coriolis gyros may be in the form of either an open loop or a closedloop system. In a closed loop system, the amplitude of the readoscillation is continuously reset to a fixed value (preferably zero) bycontrol loops.

FIG. 2 is a schematic diagram of a closed loop Coriolis gyro 1. The gyro1 has a mass system 2 that can be caused to oscillate and is referred tobelow as a resonator 2 (in contrast to the “abstract” resonators,mentioned above, which represent individual oscillations of the “real”resonator). As already mentioned, the resonator 2 may be regarded as asystem composed of two “resonators” (a first resonator 3 and a secondresonator 4). Each of the first and second resonators 3, 4 is coupled toa force transmitter (not shown) and to a tapping-off system (not shown).Noise produced by the force transmitter and the tapping-off system isindicated schematically by noise 1 (reference symbol 5) and noise 2(reference symbol 6).

The Coriolis gyro 1 includes four control loops. A first control loop isemployed for controlling the stimulation oscillation (i.e. the frequencyof the first resonator 3) at a fixed frequency (resonant frequency). Thefirst control loop has a first demodulator 7, a first low-pass filter 8,a frequency regulator 9, a VCO (voltage controlled oscillator) 10 and afirst modulator 11. A second control loop controls the stimulationoscillation at a constant amplitude and includes a second demodulator12, a second low-pass filter 13 and an amplitude regulator 14.

Third and fourth control loops are used for resetting forces thatstimulate the read oscillation. The third control loop includes a thirddemodulator 15, a third low-pass filter 16, a quadrature regulator 17and a second modulator 18. The fourth control loop comprises a fourthdemodulator 19, a fourth low-pass filter 20, a rotation rate regulator21 and a third modulator 22.

The first resonator 3 is stimulated at its resonant frequency 1. Theresultant stimulation oscillation is tapped off, demodulated in phase bymeans of the first demodulator 7, and a demodulated signal componentpassed to the first low-pass filter 8 that removes the sum frequencies.The tapped-off signal is referred to below as the tapped-off stimulationoscillation signal. An output from the first low-pass filter 8 issupplied to a frequency regulator 9 that controls the VCO 10 as afunction of the applied signal so that the in-phase componentessentially tends to zero. For this, the VCO 10 sends a signal to thefirst modulator 11, which controls a force transmitter so that astimulation force is applied to the first resonator 3. When the in-phasecomponent is zero, the first resonator 3 oscillates at its resonantfrequency ω1. It should be mentioned that all of the modulators anddemodulators are operated on the basis of resonant frequency ω1.

The tapped-off stimulation oscillation signal is also passed to thesecond control loop and demodulated by the second demodulator 12. Theoutput of the second demodulator 12 is passed through the secondlow-pass filter 13, whose output signal is, in turn, applied to theamplitude regulator 14. The amplitude regulator 14 controls the firstmodulator 11 as a function of such signal and of a nominal amplitudetransmitter 23 such that the first resonator 3 oscillates at a constantamplitude (i.e. the stimulation oscillation has constant amplitude).

As has already been mentioned, movement or rotation of the Coriolis gyro1 results in Coriolis forces (indicated by the FC·cos(ω1·t) in thedrawing) that couple the first resonator 3 to the second resonator 4,causing the second resonator 4 to oscillate. A resultant readoscillation at frequency ω2 is tapped off so that a correspondingtapped-off read oscillation signal (read signal) is supplied to both thethird and fourth control loops. In the third control loop, this signalis demodulated by means of the third demodulator 15, the sum frequenciesremoved by the third low-pass filter 16, and the low-pass-filteredsignal supplied to quadrature regulator 17 whose output is applied tothe third modulator 22 so that corresponding quadrature components ofthe read oscillation are reset. Analogously, the tapped-off readoscillation signal is demodulated in the fourth control loop by means ofa fourth demodulator 19. It then passes through a fourth low-pass filter20 and the filtered signal is applied to a rotation rate regulator 21.The output of the rotation rate regulator 21 is proportional to theinstantaneous rotation rate and is passed as the rotation ratemeasurement to a rotation rate output 24 and to the second modulator 18,which resets the corresponding rotation rate components of the readoscillation.

A Coriolis gyro 1 as described above can be operated in either adouble-resonant form or in a form in which it is not double-resonant.When the Coriolis gyro 1 is operated in a double-resonant form, thefrequency of ω2 of the read oscillation is approximately equal to thefrequency ω1 of the stimulation oscillation. In contrast, when it isoperated in a form in which it is not double-resonant, the frequency ω2of the read oscillation differs from the frequency ω1 of the stimulationoscillation. In the case of double-resonance, the output signal from thefourth low-pass filter 20 contains information about the rotation rate,while, when it is not operated in double-resonant form, the outputsignal from the third low-pass filter 16 contains the rotation rateinformation. A doubling switch 25 which selectively connects the outputsof the third and fourth low-pass filters 16, 20 to the rotation rateregulator 21 and to the quadrature regulator 17 is provided forswitching between the double-resonant and non-double resonant modes.

When the Coriolis gyro 1 is operated in a double-resonant form, thefrequency of the read oscillation is tuned, as mentioned, to that of thestimulation oscillation. This may be done to the resonator 2, forexample by mechanical means, in which material is removed from the masssystem. As an alternative, the frequency of the read oscillation can beset by means of an electrical field in which the resonator 2 is mountedto oscillate (i.e., by changing the electrical field strength). It isthus possible to tune the frequency of the read oscillation to thefrequency of the stimulated oscillation electronically during operationof the Coriolis gyro 1.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the invention to provide a method for electronicallytuning the frequency of the read oscillation in a Coriolis gyro to thatof the stimulation oscillation.

The invention addresses the preceding and other objects by providing, ina first aspect, a method for electronically tuning the frequency of theread oscillation in a Coriolis gyro. A disturbance force is applied tothe resonator of the gyro so that the stimulation oscillation remainsessentially uninfluenced and the read oscillation is changed such that aread signal that represents the read oscillation contains acorresponding disturbance component. According to the method, thefrequency of the read oscillation is controlled so that the magnitude ofthe disturbance component in the read signal is made as small aspossible.

In a second aspect, the invention provides a Coriolis gyro having arotation rate control loop and a quadrature control loop. Such gyroincludes a device for electronic tuning of the frequency of the readoscillation to that of the stimulation oscillation.

Such device includes a disturbance unit that passes a disturbance signalto either the rotation rate control loop or to the quadrature controlloop. A disturbance signal detection unit determines a disturbancecomponent contained in a read signal produced by the disturbance signal.A central unit controls the frequency of the read oscillation so thatthe magnitude of the disturbance component contained in the read signalis a small as possible.

The preceding and other features of the invention will become furtherapparent from the detailed description that follows. Such description isaccompanied by a set of drawings. Numerals of the drawings,corresponding to those of the written description, point to the featuresof the invention with like numerals referring to like featuresthroughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a Coriolis gyro based on the method ofthe invention; and

FIG. 2 is a schematic diagram of a Coriolis gyro in accordance with theprior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a schematic diagram of a Coriolis gyro 1′ based on the methodof the invention. The Coriolis gyro 1′ additionally includes adisturbance unit 26, a demodulation unit 27 and a read oscillationfrequency regulator 28.

The disturbance unit 26 generates an alternating signal of frequencyωmod that is added to the output of a quadrature regulator 21 (i.e. atthe force output from the quadrature control loop). The collated signalobtained in this way is supplied to a (third) modulator 22 whose outputis applied to a force transmitter (not shown), and, thus, to theresonator 2. As long as the frequency of the read oscillation does notessentially match that of the stimulation oscillation, the alternatingsignal produced by the disturbance modulation unit 26 is observed, after“passing through” the resonator 2, in the form of a disturbancecomponent on the tapped-off read oscillation signal.

The tapped-off read oscillation signal is subjected to a demodulationprocess (carried out by means of a fourth demodulator 19) and suppliedto a fourth low-pass filter 20 whose output is applied both to arotation rate regulator 21 and to the demodulation unit 27. The signalsupplied to the demodulation unit 27 is demodulated with a modulationfrequency ωmod that corresponds to the frequency of the alternatingsignal produced by the disturbance unit 26. The disturbance component(or the signal which represents the disturbance) is thus determined.

The demodulation unit 27 in this example can thus be regarded as adisturbance signal detection unit. An output signal from thedemodulation unit 27 is supplied to the read oscillation frequencyregulator 28 that sets the frequency of the read oscillation as afunction of it so that the output signal from the demodulation unit 27(i.e. the strength of the observed disturbance component) is a minimum.When a minimum has been reached, then the frequencies of the stimulationoscillation and the read oscillation essentially match. The signalsupplied to the demodulation unit 27 may also, as an alternative to thesignal supplied to the rotation rate regulator 21, be the signal thatthe rotation rate regulator 21 emits.

As mentioned above, and as an alternative, the alternating signalproduced by the disturbance unit 26 can also be added to an output ofthe rotation rate regulator 21. In such case, the signal supplied to thedemodulation unit 27 would be tapped off at the input or output of thequadrature regulator 17.

In principle, it is also possible to feed the disturbance signal (inthis case the alternating signal, although other disturbance signalssuch as band-limited noise are also possible) into the quadraturecontrol loop at any desired point (not only directly upstream of thethird modulator 22, i.e., at any desired point between the point atwhich the read oscillation is tapped off and the third modulator 22).Analogous considerations apply to feeding the disturbance signal intothe rotation rate control loop.

Once the Coriolis gyro 1′ has been switched on, it is advantageous toset the modulation frequency ωmod of the alternating signal to a highvalue to quickly achieve coarse control of the read oscillationfrequency. It is then possible to switch to a relatively low modulationfrequency ωmod to set resonance of the read oscillation precisely.Further, the amplitude of the modulation frequency ωmod can be greatlyreduced a certain amount of time after stabilization of the rotationrate regulator 21 and/or of the quadrature regulator 17. Since thealternating signal at the output of the rotation rate control loop,(i.e. the third control loop) is compensated, there is generally no needfor any blocking filter for the modulation frequency ωmod in therotation rate control loop.

At the same time, the rotation rate regulator 21 associates the thirddemodulator 15 and the fourth demodulator 19 in phase with the forcetransmitters for the rotation rate control loop (cosine-wave forces) andthe quadrature control loop (sine-wave forces). The rotation rate(fourth control loop) and quadrature control loop (third control loop)can thus be separated even when phase shifts occur in the analogelectronics of the Coriolis gyro 1′. These can occur, particularly as afunction of temperature. In general, a high bias will occur in thequadrature control loop. If this control loop and the rotation ratecontrol loop are not clearly separated from one another, such bias willalso appear in the rotation rate control loop.

Even when electronic frequency matching between the stimulation and theread oscillations is not desirable, the described control mechanism canbe used to insure that the quadrature control loop and the rotation ratecontrol loop are orthogonal. In this case, the controlled variable isthe reference phase of the third and fourth demodulators 15, 19, whichare respectively “responsible” for quadrature components and rotationrate components of the read oscillation. This control process ispreferably carried out digitally in a signal processor (DSP) and rendersthe Coriolis gyro insensitive to phase shifts in the analog electronics.

In a second, alternative method for electronic tuning the frequency ofthe read oscillation to that of the stimulation oscillation in aCoriolis gyro, a disturbance force is applied to the resonator of theCoriolis gyro so that (a) the stimulation oscillation remainsessentially uninfluenced, and (b) the read oscillation is changed suchthat a read signal which represents the read oscillation contains acorresponding disturbance component. In this way, the frequency of theread oscillation is controlled so that any phase shift between adisturbance signal that produces the disturbance force and thedisturbance component contained in the read signal is as small aspossible. In this case, “resonator” refers to the entire mass system (orpart of it) that can be caused to oscillate in the Coriolis gyro (i.e.,that part of the Coriolis gyro that is annotated with reference numeral2).

A significant discovery on which the second alternative method is basedis that the “time for disturbance to pass through” the resonator (i.e.,an artificial change to the read oscillation resulting from theapplication of appropriate disturbance forces to the resonator), thetime that passes from the effect of the disturbance on the resonatoruntil the disturbance is tapped off as part of the read signal, isdependent on the frequency of read oscillation. The shift between thephase of the disturbance signal and the phase of the disturbancecomponent signal contained in the read signal is thus a measure of thefrequency of the read oscillation. It can be shown that the phase shiftassumes a minimum when the frequency of the read oscillation essentiallymatches that of the stimulation oscillation. If the frequency of theread oscillation is controlled such that the phase shift assumes aminimum, then the frequency of the read oscillation is at the same timeessentially matched to the frequency of the stimulation oscillation.

In a third alternative embodiment of the method for electronic tuning ofthe frequency of the read oscillation to that of the stimulationoscillation in a Coriolis gyro, a disturbance force is applied to theresonator of the Coriolis gyro such that (a) the stimulation oscillationremains essentially uninfluenced and (b) the read oscillation is changedso that a read signal representing the read oscillation contains acorresponding disturbance component. The disturbance force is defined asthe force caused by the signal noise in the read signal. The frequencyof the read oscillation, in such case, is controlled so that themagnitude of the disturbance component contained in the read signal(i.e., the noise component) is a small as possible.

“Resonator” in this case refers to the entire mass system that can becaused to oscillate in the Coriolis gyro (i.e., that part of theCoriolis gyro which is identified by the reference number 2). Theessential feature in this case is that the disturbance forces on theresonator change only the read oscillation, but not the stimulationoscillation. With reference to FIG. 2, this would mean that thedisturbance forces act only on the second resonator 4, but not the firstresonator 3.

A significant discovery on which the third method is based is that adisturbance signal, in the form of signal noise, which occurs directlyin the tapped-off read oscillation signal or at the input of the controlloops (rotation rate control loop/quadrature control loop), can beobserved to a greater extent in the tapped-off read oscillation signalafter “passing through” the control loops and the resonator, the lessthe frequency of the read oscillation matches the frequency of thestimulation oscillation. The signal noise (the signal noise of the readoscillation tapping-off electronics or the random walk of the Coriolisgyro) is applied, after “passing through” the control loops, to theforce transmitters and thus produces corresponding disturbance forcesthat are applied to the resonator and, thus, cause an artificial changein the read oscillation. The “penetration strength” of a disturbancesuch as this to the tapped-off read oscillation signal is thus a measureof how accurately the frequency of the read oscillation is matched tothat of the stimulation oscillation. Thus, if the frequency of the readoscillation is controlled so that the penetration strength assumes aminimum, (i.e., the magnitude of the disturbance component which iscontained in the tapped-off read oscillation signal, that is the noisecomponent) then the frequency of the read oscillation is at the sametime matched to the frequency of the stimulation oscillation.

The first method described for electronic tuning of the read oscillationfrequency can be combined as required with the second method and/or withthe third method. For example, it is possible to use the methoddescribed first while the Coriolis gyro is being started up (rapidtransient response), and then to use the third method (slow controlprocess) in steady-state operation.

A major discovery on which the invention is based is that an artificialchange to the read oscillation in the rotation rate channel orquadrature channel is visible to a greater extent, in particular in therespective channel which is orthogonal to it, the less the frequency ofthe read oscillation matches the frequency of the stimulationoscillation. The “penetration strength” of a disturbance such as this tothe tapped-off read oscillation signal (in particular to the orthogonalchannel) is thus a measure of how accurately the frequency of the readoscillation is matched to the frequency of the stimulation oscillation.Thus, if the frequency of the read oscillation is controlled so that thepenetration strength assumes a minimum (i.e., such that the magnitude ofthe disturbance component which is contained in the tapped-off readoscillation signal is a minimum) then the frequency of the readoscillation is at the same time essentially matched to the frequency ofthe stimulation oscillation. The significant factor in this case is thatthe disturbance forces on the resonator change only the readoscillation, but not the stimulation oscillation. With reference to FIG.2, this means that the disturbance forces act only on the secondresonator 4, but not on the first resonator 3.

The disturbance force is preferably produced by a disturbance signalthat is supplied to appropriate force transmitters, or is added tosignals which are supplied to the force transmitters. For example, adisturbance signal can be added to the respective control/reset signalsfor control/compensation of the read oscillation, to produce thedisturbance force.

The disturbance signal is preferably an alternating signal (e.g. asuperposition of sine-wave signals and cosine-wave signals). Thisdisturbance signal is generally at a fixed disturbance frequency so thatthe disturbance component of the tapped-off read oscillation signal canbe determined by means of an appropriate demodulation process carriedout at the disturbance frequency. One alternative is to use band-limitednoise instead of an alternating signal. In this case, the disturbancecomponent is demodulated from the read signal by correlation of thedisturbance signal (noise signal) with the read signal (the signal whichcontains the disturbance component). The bandwidth of the noise in thiscase is dependent on the characteristics of the resonator 2 and of thecontrol loops.

The method described above can be used for both an open loop and aclosed loop Coriolis gyro. In the latter case, the disturbance signal ispreferably added to the respective control/reset signals forcontrol/compensation of the read oscillation. For example, thedisturbance signal can be added to the output signal from a rotationrate control loop, and the disturbance component can be determined froma signal that is applied to or emitted from a quadrature regulator in aquadrature control loop. Conversely, the disturbance signal can be addedto the output signal from the quadrature control loop, and thedisturbance component can be determined from a signal that is applied toor is emitted from a rotation rate regulator in the rotation ratecontrol loop. As an alternative, the disturbance signal can be added tothe output signal from the quadrature control loop and the disturbancecomponent determined from a signal which is applied to, or emitted from,a quadrature regulator in the quadrature control loop. It is alsopossible to add the disturbance signal to the output signal from therotation rate control loop, and to determine the disturbance componentfrom a signal which is applied to, or emitted from, a rotation rateregulator in the rotation rate control loop. The expression “readsignal” covers all signals that are referred to in this paragraph andfrom which the disturbance component can be determined. In addition, theexpression “read signal” covers the tapped-off read oscillation signal.

The frequency of the read oscillation (i.e. the force transmission ofthe control forces which are required for frequency control) is in thiscase controlled by controlling the intensity of an electrical field inwhich at least a part of the resonator oscillates, with an electricalattraction force. Such force, preferably non-linear, is establishedbetween the resonator and an opposing piece, fixed to the frame andsurrounding.

While the invention has been described with reference to itspresently-preferred embodiment, it is not limited thereto. Rather, theinvention is limited only insofar as it is defined by the following setof patent claims and includes within its scope all equivalents thereof.

1. A method for electronic tuning of the frequency of the readoscillation to the frequency of the stimulation oscillation in acoriolis gyro, wherein the resonator of the coriolis gyro has adisturbance force applied to it such that a) the stimulation oscillationremains essentially uninfluenced, and b) the read oscillation is changedsuch that a read signal, which represents the read oscillation, containsa corresponding disturbance component, wherein the frequency of the readoscillation is controlled such that the magnitude of the disturbancecomponent, which is contained in the read signal, is as small aspossible:
 2. The method as claimed in claim 1, characterized in that thedisturbance force is produced by a disturbance signal which is added tothe respective control/reset signals for control/compensation of theread oscillation.
 3. The method as claimed in claim 1, characterized inthat the disturbance signal is an alternating signal.
 4. The method asclaimed in claim 3, characterized in that the disturbance signal is at afixed disturbance frequency, and the disturbance component is determinedfrom the read signal by demodulation of the read signal at the fixeddisturbance frequency.
 5. The method as claimed in claim 1,characterized in that the disturbance signal is band-limited noise, andthe disturbance component is demodulated from the read signal bycorrelation of the disturbance signal with the read signal.
 6. Themethod as claimed in claim 2, characterized in that the disturbancesignal is added to the output signal from the rotation rate controlloop, and the disturbance component is determined from a signal which isapplied to a quadrature regulator in the quadrature control loop, or isemitted from it.
 7. The method as claimed in claim 2, characterized inthat the disturbance signal is added to the output signal from thequadrature control loop, and the disturbance component is determinedfrom a signal which is applied to a rotation rate regulator in therotation rate control loop, or is emitted from it.
 8. The method asclaimed in claim 2, characterized in that the disturbance signal isadded to the output signal from the quadrature control loop, and thedisturbance component is determined from a signal which is applied to aquadrature regulator in the quadrature control loop, or is emitted fromit.
 9. The method as claimed in claim 2, characterized in that thedisturbance signal is added to the output signal from the rotation ratecontrol loop, and the disturbance component is determined from a signalwhich is applied to a rotation rate regulator in the rotation ratecontrol loop, or is emitted from it.
 10. The method as claimed in claim2, characterized in that the frequency of the read oscillation iscontrolled by controlling the intensity of an electrical field in whicha part of the resonator of the Coriolis gyro oscillates.
 11. A Coriolisgyro which has a rotation rate control loop and a quadrature controlloop, characterized by a device for electronic tuning of the frequencyof the read oscillation to the frequency of the stimulation oscillation,having: a disturbance unit which passes a disturbance signal to therotation rate control loop or to the quadrature control loop, adisturbance signal detection unit, which determines a disturbancecomponent which is contained in a read signal (which represents the readoscillation) and has been produced by the disturbance signal, and acontrol unit, which controls the frequency of the read oscillation suchthat the magnitude of the disturbance component, which is contained inthe read signal, is as small as possible.
 12. The Coriolis gyro as claim11, characterized in that the disturbance unit passes the disturbancesignal to the rotation rate control loop, and the disturbance signaldetection unit determines the disturbance component from a signal whichis applied to a quadrature regulator in the quadrature control loop, oris emitted from it.
 13. The Coriolis gyro as claim 11, characterized inthat the disturbance unit passes the disturbance signal to thequadrature control loop, and the disturbance signal detection unitdetermines the disturbance component from a signal which is applied to arotation rate regulator in the rotation rate control loop, or is emittedfrom it.
 14. The Coriolis gyro as claim 11, characterized in that thedisturbance unit passes the disturbance signal to the rotation ratecontrol loop, and the disturbance signal detection unit determines thedisturbance component from a signal which is applied to a rotation rateregulator in the rotation rate control loop, or is emitted from it. 15.The Coriolis gyro as claim 11, characterized in that the disturbanceunit passes the disturbance signal to the quadrature control loop, andthe disturbance signal detection unit determines the disturbancecomponent from a signal which is applied to a quadrature regulator inthe quadrature control loop, or is emitted from it.
 16. The Coriolisgyro as claimed in claim 11, characterized in that the disturbancesignal is an alternating signal at a fixed disturbance frequency, andthe device for electronic tuning of the read oscillation frequency andstimulation oscillation frequency has a demodulation unit, whichdemodulates the read signal at the fixed disturbance frequency and thusdetermines the disturbance component which is contained in the readsignal.